There have been known, as magnetic resonance measuring apparatuses, a nuclear magnetic resonance (NMR) measuring apparatus and an electron spin resonance (ESR) measuring apparatus. As an apparatus categorized as the NMR measuring apparatus, a magnetic resonance imaging (MRI) apparatus has also been known. The NMR measuring apparatus will be explained below.
Nuclear magnetic resonance is a phenomenon in which atomic nuclei placed in a static magnetic field interact with an electromagnetic wave having a frequency specific to the atomic nuclei. An apparatus that utilizes the phenomenon for measuring a test sample at an atomic level is referred to as an NMR measuring apparatus. The NMR measuring apparatus has been used for analyzing materials, including organic compounds (such as, for example, chemical agents and agricultural chemicals), polymeric materials (such as, for example, vinyl and polyethylene), biological materials (such as, for example, nucleic acid and protein). Using the NMR measuring apparatus, for example, the molecular structure of a sample can be identified.
In general, the NMR measuring apparatus includes a control computer, an RF signal transmitter, an NMR signal detector (probe), a static magnetic field generator (a superconductive magnet), an NMR signal receiver, and other components. In some cases, however, the NMR measuring apparatus may refer to a part of the NMR measuring apparatus including some of the above-listed components. For example, a part corresponding to a spectrometer including the control computer, the RF signal transmitter, and the NMR signal receiver may be referred to as the NMR measuring apparatus. In typical NMR measurement, a radio frequency signal (RF transmission signal) used for the NMR measurement is generated in the transmitter and supplied to a transmitting and receiving coil in a probe. This generates an electromagnetic wave that causes a resonance absorption phenomenon in nuclei to be observed within the sample. Then, an NMR signal (RF reception signal) induced in the transmitting and receiving, coil is sent to the receiver, and a spectrum of the received signal is analyzed.
Dynamic nuclear polarization (DNP) is known as a method for amplifying the strength of the NMR signal. In the DNP method, a substance (radical) including an unpaired electron is added to the sample, and a mixture of the radical and the sample is irradiated with a microwave to excite electron spin resonance. Through this excitation, a high degree of polarization of a spin of the unpaired electron is transferred to polarization of a nuclear spin, which leads to an approximately thousand-fold increase in the strength of the NMR signal.
The strength of the NMR signal obtained with the DNP method greatly depends on magnetic relaxation of the radical added to the sample. When the length of radical relaxation time is shorter, a magnetized state of the radical is attenuated before the polarization of the nuclear spin is enhanced, resulting in a lesser extent of the increase in strength of the NMR signal. It has been known that as the temperature becomes lower, the radical relaxation time becomes longer. For example, at temperatures of liquid nitrogen or below, the radical relaxation time is increased to a length substantially equal to or greater than the length of time to transfer magnetization between the electron spin and the nuclear spin. It is therefore expected that the strength of the NMR signal can be dramatically increased at the temperature of liquid nitrogen or below.
In an NMR apparatus described in JP 2010-523204 A, a sample is cooled by immersing the sample in liquid helium. Helium gas vaporized from the liquid helium is recondensed to thereby regenerate liquid helium for reuse.
In an NMR probe disclosed in JP 2008-241493 A, helium gas vaporized from liquid helium is used to cool down a transmitting and receiving coil, a variable capacitor, and a preliminary amplifier. In addition, a bearing gas and a driving gas are supplied to a sample rotor in order to rotate a sample rotor.
On the other hand, helium gas is circulated for cooling a transmitting and receiving coil and a tuning and matching circuit in an NMR probe described in JP 2004-219361 A.
When such a DNP method is applied, it is desirable that the sample be effectively cooled while avoiding a temperature rise in the sample as far as possible, to thereby prolong the radical relaxation time. When the sample is a solid substance, the sample rotor is typically rotated in an inclined position at a predetermined angle of inclination (at a so-called “magic angle”). The apparatus described in JP 2010-523204 A includes no mechanism for rotating the sample rotor, and is therefore unsuitable for use in measurement of a sample which should be rotated. Meanwhile, in the NMR probes described in JP 2008-241493 A and JP 2004-219361 A having no mechanism for cooling a sample, it is almost impossible to improve detection sensitivity in the DNP method.